During the past year a research program was initiated to biochemically characterize retroviruses isolated from patients with the lymphadenopathy or the acute immunodeficiency syndromes (AIDS). A virus stack (LAV) and detailed protocols for stimulating human lymphocytes and propagating lymphotropic human retroviruses were kindly provided by Dr. Luc Montagnier, Pasteur Institute, Paris. The initial virus stocks were titrated and propagated in stimulated normal human lymphocytes and lymphotropic retrovirus pools were prepared. We have been successful in isolating a lymphotropic retrovirus from a patient with the pre-AIDS syndrome and are preparing high-titer pools of this new inoculum. Lymphotropic retrovirus stocks have been propagated in a continuous cell line (CEM) yielding virus titers comparable to those obtained with stimulated normal human lymphocytes. We are currently in the process of molecular cloning lymphotropic retrovirus proviral DNAs from infected cells. Molecular clones obtained will be used in two types of experiments. Constructs will be made utilizing the LTR and other gene segments to introduce a variety of DNAs into subsets of T-helper lymphocytes. In vitro mutagenesis will be carried out to ascertain the principal determinants of host range. In a second group of experiments lymphotropic retroviral gene segments will be inserted into a variety of expression vector systems to elicit the synthesis of novel viral proteins.